A particularly desirable use of the present invention is in connection with the production of textile items made from terry cloth.
Terry fabric is produced by automatic machines and is supplied in rolls or spools containing rolled lengths of fabric which may be several hundred meters long. Each roll consists of multiple segments of plush surface of tufted fabric which are separated by grooves of untufted bands. Each segment may have design features incorporated on its surface such as printed pattern or decorative grooves an marks, which are according to buyer's taste and fashion demands. Although the designer intention is to have uniform segments, the length and the shape of segments within the roll may vary considerably according to stretching force, dyeing process, humidity and elasticity of treads.
In the past apparatus have been built and used for detection of a change in thickness of the terry fabric. A typical illustration of such apparatus can be found in U.S. Pat. No. 3,182,147. The apparatus includes two double-arm levers, each of which is mounted to a shaft. A sensing arm of a lower lever slides on the surface of the fabric and its motion is mechanically multiplied by the upper lever. The magnified motion activates a mechanical switch which provides an electrical connection. A similar prior apparatus but with one double-arm lever is disclosed in U.S. Pat. No. 4,375,175.
Another prior type of sensing device is disclosed in U.S. Pat. No. 4,187,132. This device uses mechanical scanners which are movable in vertical direction under the influence of the differences in height level of the fabric moving below. The scanners close an electrical contact when they detect a groove.
Such prior devices have sensing arms mounted rotatably to a shaft and do not have means to distinguish between small irregularities in the fabric thickness, such as a knot or a thicker thread, and therefore often incur false readings. In addition, prolonged use of such sensing arms results in recurring misalignment and the need for frequent adjustment.
The terry fabric has usually symmetrical grooves on both sides which are stretched during the sensing process. In case of narrow symmetrical grooves, the tension may be high and the groove on one side may not contact the base plate of the prior type of machine. In such circumstances the prior devices will not be able to sense the groove. This can be a serious problem, since the depth of a groove may be as small as 0.14 millimeter and therefore not detectable by the prior devices. If a strong spring is used to ensure contact of the sensing finger with the terry fabric, its pressure may crush the terry and cause false readings. Therefore, the stretching devices available heretofore have not provided reliable means to detect grooves in terry fabric.
Another prior apparatus is disclosed in U.S. Pat. No. 3,182,536 which discloses the use of two rollers mounted on opposite sides of the terry fabric. When a groove comes between the rollers, they get closer, and the change in their mutual position is transmitted through a set of levers which intercept a light beam to represent the presence of the groove. The apparatus is large and has a considerable mechanical inertia. In addition, it is not applicable to detection of narrow grooves because it uses long rollers which cannot engage narrow grooves which have been distorted by web or weft bias in the fabric.
Another prior apparatus is disclosed in U.S. Pat. No. 2,637,115. That apparatus includes a permanent magnet which is positioned on one side of a non-magnetic material and a movable ferromagnetic piece of metal which is positioned on the other side. The piece of metal is attracted by the magnet and the change in its position is proportional to the thickness of the material. A signal corresponding to the change in thickness is produced by a mechanical switch which is connected to the movable finger. This system is not sensitive enough for detection of small grooves in terry fabric because of the mechanical hysteresis of the switch.
Another prior apparatus is disclosed in the U.S. Pat. No. 4,364,502. That patent discloses a rotating set of magnetic sensors which detect the position of a ferromagnetic line in a web of material to be cut. That method is applied to cut a long piece of carpet which contains a line of ferromagnetic iron oxide or iron powder in its surface. A similar prior method of sensing a ferromagnetic line in fabric is disclosed in U.S. Pat. No. 4,493,234. However, terry fabric and many other non-magnetic materials do not have ferromagnetic material incorporated in the surface and, therefore, such methods are not applicable.
Another prior apparatus is disclosed in the U.S. Pat. No. 3,553,668. That apparatus uses a change of capacitive or inductive properties between two plates to detect irregularities in the thickness of a moving web. The plates are attached to rollers which are in contact with the web. If a local surface elevation passes under one of the rollers, it produces a mutual displacement between the rollers which changes capacitive and inductive properties between the plates. However, there is no mutual displacement if all rollers sink into a groove simultaneously. For that reason this method is not applicable for detection of grooves in terry fabric.
Another prior method incorporates a light beam for detection of thickness. The light beam is emitted by a light transmitter and directed through the material towards a light receiver. A thin band of untufted fabric lets the beam go through, while the thicker plush part stops it. However, this method is likely to provide an inaccurate signal if applied to terry fabric because the tufted surface of terry cloth has openings through which the light beam can penetrate and the untufted bands are often dense enough to stop such a light beam.
Although, it has been recognized heretofore that it is desirable to automate the work stations for terry fabric, only the towel terry without grooves or with relatively wide grooves has been cut into segments by means of automatic machines. Terry fabric with narrow grooves, usually less than 6 millimeters, has heretofore been cut by hand because of the difficulties of detecting narrow grooves and eliminating the bias in the fabric. The worker had to move the terry fabric along a table, locate a groove between plush areas and then cut the fabric in the middle of the groove using a rotatable cutting disk. This is a slow and unsafe operation which requires skilled labor. In addition, the manual cutting quite often results in improper cuts because of the difficulty of cutting exactly in the middle of a distorted and narrow groove.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for detecting surface discontinuities in non-magnetic material such as the transverse untufted bands or grooves of terry cloth, irrespective of the relative width of such discontinuites.
Another object of the present invention is to provide a reliable and cost-effective means for detecting the untufted bands or grooves of fabric such as terry fabric.
Still another object of the present invention is to provide a method and apparatus for detecting surface discontinuities and which can be employed as an integral part of a control system in any automated machine which processes non-magnetic material.
A further object of the present invention is to provide a method and apparatus for detecting surface irregularities and which may be employed in connection with automated fabric stretching equipment of the type described in my co-pending application Ser. No. 07/176,423. The text of said application is incorporated herein by reference.